Mechanical switches are arranged in pairs of switches and must be operated in a specific combination to operate or stop the DC motor. For example, switch combination A+D controls forward rotation, while switch combination B+C Inversion of control, as shown in the figure. Switch combinations A+B or C+D can cause a short circuit at the motor terminals, resulting in rapid braking of the motor. However, using switches in this way also carries its own dangers, as operating switches A+C or B+D simultaneously can cause a short circuit in the power supply.

    Although the above two circuits can work well for most small DC motor applications, do we really want to operate different combinations of mechanical switches to reverse the direction of the motor? No! We can change the manual switches of a set of electromechanical relays and use a single forward reverse button or switch, or even use a solid-state CMOS 4066B quad bilateral switch.

    But another very good way to achieve bidirectional control of the motor (and its speed) is to connect the motor to a transistor H-bridge circuit arrangement, as shown below.